The document discusses modal verbs used to express possibility and probability when speculating about who is at the door after the doorbell rings. "Must" expresses certainty that it is the postman, "can't" expresses certainty that it is not the postman, while "might" and "could" both express that it is possible but not certain that it is the postman. The modals "might" and "could" indicate a possibility or probability, while "must" and "can't" denote certainty or impossibility.

1. MODALS OF POSSIBILITY

2. IMAGINE YOU ARE AT HOME, RELAXING YOURSELF... WHEN SUDDENLY, THE DOORBELL RINGS

3. You can’t see who is at the door

4. YOU SAY... That must be the postman That might be the postman That can't be the postman That could be the postman This

5. But what’s the difference?

6. THAT must BE THE POSTMAN This means you are CERTAIN it is the postman

7. THAT can’t BE THE POSTMAN This means you are CERTAIN that it is not the postman.

8. THAT might BE THE POSTMAN This means you are NOT CERTAIN it is the postman, but it is probable or possible.

9. THAT could BE THE POSTMAN This means you are also NOT CERTAIN it is the postman but it is possible or probable.